1. Field of the Invention
The present invention relates to a process for transmitting data packets from mobile stations to base stations in mobile radio systems operated by the time-division multiplex method.
2. Description of the Prior Art
For the transmission of data between two communication terminals, recourse can be made to connection-oriented concepts and concepts based on virtual (logical) links. In the case of connection-oriented data transmissions, physical resources have to be provided between the two communication terminals during the entire time of the data transmission.
In the case of data transmission via logical links, the permanent provision of physical resources is unnecessary. An example of such data transmission is packet data transmission. Here, a logical link between the two communication terminals exists during the entire duration of the data transmission, but physical resources are provided only during the actual transmission times of the data packets. This process is based on the fact that the data is sent in short data packets, between which longer pauses can occur. In the pauses between the data packets, the physical resources are available for other logical links. With respect to one logical link, physical resources are saved.
The packet data transmission process is suitable in particular for communication systems with limited physical resources, whenever short data packets have to be transmitted only sporadically. For example in mobile radio systems, such as the GSM mobile radio system, the physical resources are restricted in terms of frequency range (number of frequency channels) and time slots. Accordingly, such physical resources have to be used efficiently.
The GSM mobile radio system operates by the time-division multiplex method, with the result that time slots within a frequency channel can be divided between various communication terminals. In a communication relationship between a base station, which is further connected to a communication network, and a mobile station, it must be ensured that the information items sent by the various mobile radio stations arrive in the base station in conformity with the time slots. Since the distance, and consequently the delay, between mobile station and base station may well differ and/or changes if the mobile station moves, it is necessary to calculate a timing advance which influences the time of sending for the respective time slot in the mobile station.
The GSM system was conceived for voice transmission. In a voice transmission, information items are constantly exchanged between mobile station and base station. There is, therefore, no difficulty in determining the timing advance sufficiently frequently on the basis of the constantly exchanged information items.
However, the amount of signalling which voice transmission involves, and which can be used also for the determination of the timing advance, is inappropriately high, for packet data transmission. A process for packet data transmission in which the timing delay is determined before each transmission of the data packet is known from Telia AB/Telia Research, from the GPRS Conference from 10 to Jan. 12, 1995. As emerges from page 8, items 3 and 4 of the updating procedure, the mobile station sends an access data packet on an access channel GACH, whereupon the base station determines the timing advance and sends the determined timing advance to the mobile station via an acknowledgement channel GCCH. The timing advance must consequently still be determined relatively frequently. Also, accesses to the access channel and acknowledgement channel are necessary in this case, or these channels have to be separately provided.